As the demand for natural oil and gas increases, so does the need for efficient retrieval of these limited resources from their subterranean positions. Hence, through an abundance of research and development, the techniques and equipment employed to remove these substances have become increasingly sophisticated and efficient. Production of underground fluid usually includes drilling of a well into a formation containing the desired fluid and then removing the fluid therefrom, usually by a mechanical or jet pump. In some instances, such as in the Middle East, sufficient bottom hole pressure is available in the formation to force the production fluid to the surface, where it is collected and utilized for commercial purposes. When natural lifting of the well is insufficient to deliver the production fluid, however, it is necessary to deploy a pump, such as a sucker rod-type or hydraulic-type pump, to lift the production fluid from the formation.
Recent developments have favored the use of hydraulic or down-hole jet pumps over mechanical-type, sucker-rod pumps. Briefly, jet pumps and hydraulic pumps generally include a power fluid line operably coupled to the entrance of the pump, and a return line coupled to receive fluids from a discharge end of the pump. As the pressurized power fluid is forced, by a pump at the surface, down through the down-hole pump, the power fluid draws in and intermixes with the production fluid, which then is recovered with the power fluid through the return line.
Typically, after a well has been drilled, a steel tubular casing, extending the length of the well, is lowered down into the well. Subsequently, uncured concrete is pumped down the casing and forced out of the bottom of the casing and up the outside of the casing into an annulus formed between an outer surface of the casing and drilled formation walls of the well. The concrete then cures to firmly anchor the casing to the well walls and seal off the well.
To access the formation fluids through the now sealed well casing, both the casing and the concrete are perforated at a predetermined down-hole location well below both the slurry plug level and the formation fluid level. These perforations allow the production fluid to enter the well casing from the formation for retrieval using the pump. Due to the difference in pressure between the formation and the well casing interior, the in-rush of the production fluid into the well casing is substantial enough to clean the perforation passages of any debris for unobstructed passage of production fluid into the well casing. As mentioned, however, the predetermined slurry plug remaining in the well above the perforations prevents blowout of the in-rushing formation fluids.
To form the casing perforations at the remote down-hole location, a perforation gun assembly capable of piercing the casing is suspended from a bottom hole assembly of the pump. Detonation or triggering of the gun assembly causes projectiles in the gun to be propelled outwardly through the casing, the concrete annulus and into the formation. As mentioned, this causes an insurgence of fluids into the well casing at a force sufficient to clear the perforation holes of any debris.
Perforation gun assemblies can be triggered by operator-induced pressure differentials or automatically triggered or detonated by formation-induced pressure differentials. Triggering in either case is based upon use of a pressure sensor assembly which is capable of sensing a predetermined pressure and/or predetermined pressure differential between the pump discharge pressure and the formation pressure or well casing pressure.
Typically, the pressure sensor assembly of the perforation gun is located at or on the perforation gun. Thus, it is necessary to communicate the pump discharge pressure to the pressure transducer located on the gun assembly. A trigger bypass tube extends inside the well casing from a position in fluid communication with the discharge end of the pump all the way down to the gun assembly, which sometimes is a distance exceeding 1000 feet. Hence, the pressure at the discharge end of the pump is transmitted in the bypass tube to the pressure sensor on the perforation gun.
When operator-induced triggering is employed, the initial operation of the pump, before the perforation of the well casing by the gun assembly, removes any production fluid (and excess drilling mud) from the well casing until the slurry plug decreases to a predetermined height. At this time, the operator closes off the power fluid line, and redirects the power fluid, pressurized from a ground level pumping station, down the return line. The power fluid in the return line passes through the pressure bypass conduit to the pressure sensor at the perforation gun. The pressure transducer at the perforation gun is exposed to and senses the rise in pressure caused by pumping of power fluid into the bypass conduit. When a pre-set pressure level is reached, the sensor triggers the perforation gun. This pre-set predetermined trigger pressure for operator-induced triggering is typically much higher than the normal production or discharge pressure generated by the pump to ensure that inadvertent triggering does not occur. For formation-induced triggering, the perforation gun assembly is automatically discharged upon the pressure sensor assembly sensing a predetermined differential pressure between the pump discharge and the formation pressure. As the level of the production fluid drops, the production fluid pressure inside the well casing decreases, thereby increasing the pressure differential relative to the pump discharge pressure. When the pressure differential between the pump and the formation reaches a predetermined level, the pressure sensor assembly triggers the gun automatically.
In either instance (i.e., operator-induced or automatic formation-induced triggering), due to the extreme violence of the detonation, the stainless steel pressure transmitting bypass tube is generally severed. Because of the positive discharge pressure from the jet pump, the production and power fluids being pumped will be discharged through the severed bypass tube, which returns a measurable portion of the overall produced fluids back into the formation. This is especially inefficient and costly since the fluids will have to be recovered again. Moreover, the constant stream of ejected production fluid from the bypass tube may prematurely erode or damage the casing and the well components over time.